The present invention describes the use of poly-o-hydroxy amides (PHAs) for adhesively bonding articles or materials, especially components used in the semiconductor industry, such as chips and wafers, a process for adhesively bonding materials, especially chips and/or wafers, chip and/or wafer stacks produced by the process, and adhesive compositions which comprise the poly-o-hydroxy amides of the formula (I).
There is an increasing requirement for high-grade adhesives especially in electronics and microelectronics, mechanical engineering, automotive engineering, and also aerospace. In many cases the critical factor is that these adhesives must withstand the extremely high thermal and chemical loads without loss of bond strength or of physical properties. It is also important that they absorb very little water, if any, since water absorption may lead at low temperatures to stresses and cracks, and at high temperatures gives rise to blistering.
The application and testing temperatures here may amount, for example, to between xe2x88x9280xc2x0 C. to +450xc2x0 C. Particularly in micro-electronics (processes at up to 450xc2x0 C.) and automotive engineering (adhesive bonds directly in the engine or transmission area), very high temperatures occur. Adhesive bonds in the area of microelectronics, chemical plant, and in the engine area necessitate very high stability of the adhesives toward solvents, acids, bases and/or aggressive gases. At the present time there is a lack in particular of adhesives which meet the requirements specified above and which are highly suitable for bonding a variety of materials, such as silicon, metal, glass, stone and/or ceramic.
In the field of microelectronics and the semiconductor industry the stacking of chips (ICs, integrated circuits) is significant owing, for example, to the increase in memory capacity, since through stacking it is possible to raise the memory capacity without increasing the area of the chip. The stacking technique is especially significant for the combination of different chips; for example, of memory chips and logic chips. Thus during the processing of the silicon wafer it is possible to carry out cost-effective production of only one kind of chips, which are later stacked atop one another and electrically contacted.
In accordance with the state of the art, materials, including chips and/or wafers, are adhesively bonded, for example, by using polyimide adhesives (C. Feger, M. M. Khojasteh, J. E. McGrath, Polyimides: Materials, Chemistry and Characterization, Elsevier Science Publishers B. V., Amsterdam, 1989, p. 151 ff.). Although the polyimides exhibit good temperature stability, the presence of the keto groups means that they absorb a relatively large amount of water, leading to the problems referred to above. Moreover, the adhesion of polyimides to many materials used in particular in microelectronics and optoelectronics is poor.
In the field of microelectronics, polyimides have been used, for example, as follows:
A polyimide is applied to the first wafer, dried, and baked in an oven at about 400xc2x0 C. The surface of the polyimide layer is then activated in a plasma (argon, oxygen). This wafer is then bonded with a second, likewise plasma-activated, wafer, with the activated sides facing one another. The second wafer can, but need not necessarily, have a polyimide layer. The great disadvantage of this process is that the bonding must be performed within about one hour following activation, since otherwise the surfaces become deactivated. Moreover, owing to the presence of the carbonyl groups, polyimide may absorb water, which may lead to blistering later on when the stack is subjected to temperature. This greatly restricts the usefulness of the process. For chip-on-wafer applications it is practically impossible to employ this process, since in general up to three hours may be needed for the bonding of the chips to a wafer, especially in the case of 200 nm and 300 nm wafers.
EP 807 852 B1 discloses compositions which comprise polyhydroxy amides, a diazoquinone compound, and a phenolic compound and/or an organosilicon compound. In the tests which were carried out here, the adhesiveness fell substantially when the phenol compound or organosilicon compound was removed from the composition or was not present in the amounts stated.
The operation of chip and/or wafer bonding requires a highly temperature-resistant and chemical-resistant adhesive bond, since a stack of this kind and hence the adhesive may come into contact with aggressive solvents and gases. Moreover, the temperatures are frequently up to 450xc2x0 C., in the case of tungsten CVD depositions, for example. The adhesive used must absorb very little water, if any, since otherwise there will be blistering at high temperatures and it may in some cases not be possible to produce the contacts reliably.
One objective of the present invention is to provide a facility for the reliable and permanent adhesive bonding of identical or different materials which are subjected to high thermal and chemical loads.
A further objective of the present invention is to provide permanent, chemical-stable and temperature-stable adhesive bonds between wafers and/or chips.
The present invention relates according to Claim 1 to the use of poly-o-hydroxy amides (PHAs) of the following formula (I) for adhesive bonding: 
where a and d independently of one another are 0 or 1, b=0-100, and c=0-100;
where X if a or d=0, i.e., if X is attached to NH, has the following definition: xe2x80x94H, alkylcarbonyl, alkenylcarbonyl, cycloalkenylcarbonyl, arylcarbonyl, aralkylcarbonyl, aralkenylcarbonyl, aralkynylcarbonyl or heterocycloalkylcarbonyl each unsubstituted or substituted, it being possible for the carbonyl group to be attached to the aromatic moiety, heterocycle, or alkyl, alkenyl or alkynyl group;
and X if a or d=1, i.e., if X is attached to CO, has the following definition:
hydroxy, substituted or unsubstituted alkoxy, alkenoxy, aryloxy, cycloalkenoxy, amino, alkylamino, alkenylamino, arylamino, arylalkenoxy, arylalkylamino;
Y1 to Y4 independently of one another have the following definition:
substituted or unsubstituted aryl, a substituted or unsubstituted polynuclear aromatic hydrocarbon compound, a substituted or unsubstituted fused ring system, or alkyl, alkenyl, alkynyl, aralkyl, aralkenyl, aralkynyl, heterocyclo or cycloalkenyl each unsubstituted or substituted;
Z1 to Z3 have the following definition, where Z1 to Z3 can be identical to or different than one another:
aryl, aralkyl, aralkenyl, aralkynyl, heteroaryl, a polynuclear aromatic hydrocarbon compound or a fused ring system each unsubstituted or substituted.
The present invention further relates to a process for adhesively bonding materials or components, wherein:
a) poly-o-hydroxy amides of the present invention are applied to the area or areas of the materials and/or components to be bonded;
b) the areas to be bonded are contacted with one another;
c) the poly-o-hydroxy amides are crosslinked, and
d) then, optionally, baking takes place in order to convert the poly-o-hydroxy amides into the corresponding polybenzoxazoles.
The invention further embraces articles and components which have been adhesively bonded by the process of the invention.
The present invention also relates to an adhesive composition comprising, based on the overall composition:
10-45% by weight of PHAs of the general formula (I),
55-90% by weight of an organic solvent, and optionally
0.1-10% by weight of a crosslinker.
In accordance with the present invention poly-o-hydroxy amides are used for adhesively bonding materials and/or components, particular preference being given to their use for bonding chips and/or wafers and, in general, articles or materials which are used in microelectronics and optoelectronics.
Particularly preferred among the PHAs of the formula (I) indicated above are:
compounds wherein
b=1-20 and/or c=0-20, with further preference c=1-20.
Also particularly preferred are:
compounds where X, if a or d=0, i.e., if X is attached to NH, is: 
where e=0-10;
R3 is: xe2x80x94H, and also:
xe2x80x94(CH2)kxe2x80x94CH3 (k=0-10) xe2x80x94(CF2)kxe2x80x94CF3 (k32 0-10) 
W is:
xe2x80x83xe2x80x94H xe2x80x94CN xe2x80x94CH(CH3)2 xe2x80x94C(CH3)3
xe2x80x94(CH2)exe2x80x94CH3 xe2x80x94(CF2)exe2x80x94CF3 xe2x80x94Qxe2x80x94(CH2)e xe2x80x94CH3 xe2x80x94Qxe2x80x94(CF2)exe2x80x94CF3 
where e=0-10;
and Q is: xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94NHxe2x80x94.
Compounds wherein X, if a or d=1, i.e., if X is attached to CO, is: 
where e=0-10;
and W is as defined above.
Compounds wherein Y1 to Y4 independently of one another are: 
where Q is: xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94NHxe2x80x94;
R1 is: xe2x80x94H, xe2x80x94CF3, xe2x80x94OH, xe2x80x94SH, xe2x80x94COOH, xe2x80x94N(R4)2, alkyl, aryl, heteroaryl, and also: 
where e=0-10;
Q is: xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94NHxe2x80x94;
R2 is: xe2x80x94Oxe2x80x94, xe2x80x94COxe2x80x94, xe2x80x94NR3xe2x80x94, xe2x80x94Sxe2x80x94, xe2x80x94SO2xe2x80x94, xe2x80x94S2xe2x80x94, xe2x80x94CH2xe2x80x94, or: 
R3 is: xe2x80x94H , and also:
xe2x80x94(CH2)kxe2x80x94CH3 (k=0-10) xe2x80x94(CF2)kxe2x80x94CF3 (k=0-10)

and R4 is: alkyl having from 1 to 10 carbon atoms, aryl or heteroaryl.
Suitable examples of polynuclear aromatic hydrocarbon radicals, fused ring systems, and heterocyclic compounds for Y1 to Y4 are biphenyl, anthracene, naphthalene, fluorene, pyrene, thiophene, thiazole or benzothiazole, imidazole or benzimidazole, pyrrole, furan, pyridine or pyrazine or derivatives thereof.
Compounds wherein Z1 to Z3 independently of one another are: 
where Q is: xe2x80x94Oxe2x80x94, xe2x80x94Sxe2x80x94 or xe2x80x94NHxe2x80x94;
and R2 is as defined above.
Suitable examples of polynuclear aromatic hydrocarbon radicals, fused ring systems, and heterocyclic compounds for Z1 to Z3 are biphenyl, anthracene, naphthalene, fluorene, pyrene, thiophene, thiazole or benzothiazole, imidazole or benzimidazole, pyrrole, furan, pyridine or pyrazine or derivatives thereof.
Particular preference is also given to compounds wherein Z1, Z2 and/or Z3 are: 
Particular preference is further given to compounds wherein Y1, Y2, Y3 and/or Y4 are: 
It is preferred for the polyhydroxy amides of the present invention to contain no constituents other than solvents and/or crosslinker substances. In particular, the polyhydroxy amides of the present invention preferably do not include the following constituents which are mandatorily present in the composition according to EP 807 852 B1 : (B), i.e., photosensitive diazoquinone compounds, (C), i.e., the phenolic compounds specified therein, and (D), i.e., the organosilicon compounds specified therein. It is preferred to use a composition which consists of the polyhydroxy amides described in accordance with the invention, an appropriate solvent, and, optionally, crosslinker substances.
Following crosslinking, optionally, the poly-o-hydroxy amides of the present invention are converted on baking into the corresponding polybenzoxazoles. However, the invention does not envisage using polybenzoxazoles themselves as adhesives. Polybenzoxazoles dissolve only relatively poorly, and are not soluble in many organic solvents. PHAs, especially the PHAs of the formula (I), on the other hand, are distinguished by very good solubility in a large number of organic solvents, including highly volatile organic solvents, so that the application of the PHAs to the surface of the articles to be bonded by spincoating, spraying, spreading or brushing techniques, in contradistinction to polybenzoxazoles, with a very large number of organic solvents, especially highly volatile solvents, is particularly favorable.
The polyhydroxy amides of the invention can be prepared by processes which are known per se. Conventionally, a bisaminophenol for introducing the group Z1, Z2 and/or Z3 is first reacted with a dicarbonyl chloride for introducing the group Y1, Y2, Y3 and/or Y4. The product obtained is then reacted with a compound for introducing the terminal group X, e.g., an X-carbonyl chloride or an anhydride. For polyhydroxy amides where a and/or d=1, X is introduced, for example, in the form of an alcohol Xxe2x80x94OH or amine Xxe2x80x94NH2.
The poly-o-hydroxy amides of the present invention can be used with preference for permanent bonding of the following materials and/or articles or articles with surfaces of the following materials: aluminum, titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, steel, brass, palladium, silver, tin, tantalum, tungsten, platinum, gold, lead, carbon, including layers containing carbon and deposited by means of plasma, carbon fibers, silicon or germanium.
Further-preferred materials are alloys of the abovementioned materials and compounds of the abovementioned materials with oxygen and/or nitrogen, especially silicon carbide, silicon nitride, silicon oxide, titanium nitride, tantalum nitride, silicon oxynitride, tungsten nitride, gallium arsenide, gallium nitride, gallium indium phosphite, indium-tin oxide.
Fundamentally, particular preference is given in accordance with the invention to adhesively bonding those compounds which are employed in microelectronics and optoelectronics.
Further materials suitable for adhesive bonding in accordance with the invention include ceramics, glass ceramics, glasses, clayware, porcelain, stoneware and/or silicates. Glasses which can be used with preference include quartz glass, soda glass, potassium glass, soda-potassium-lime glass, boron-alumina glass, borosilicate glass, and potassium-lead glass. Enamel can likewise be bonded with preference in accordance with the invention.
Furthermore, various minerals, such as marble, basalt, limestone, granite, and concrete, can be bonded with preference in accordance with the invention.
All of said materials can be adhesively bonded to themselves or to another of the abovementioned materials.
The parts to be bonded to one another in accordance with the invention withstand temperatures in the range of between xe2x88x9280xc2x0 C. to 400xc2x0 C., in the majority of cases up to 450xc2x0 C. or higher, without any problem of detachment. This is favored by the very low water absorption of the poly-o-hydroxy amides used in accordance with the invention. There is no decrease in bond strength following multiple exposure within this temperature range. Additionally, following the action of solvents, chemicals or aggressive gases on the bonded parts, including the adhesive, the bond strength is not substantially impaired. It is a particular advantage of the present invention that poly-o-hydroxy amides of the general formula (I) can be used universally for the permanent and reliable adhesive bonding of a variety of materials, with the bonds being extremely stable toward temperature and chemical exposures.
The present invention is especially suitable for the adhesive bonding of chips and/or wafers. This is carried out with the aim of stacking chips (integrated circuits), since this makes it possible to increase the memory capacity without increasing the area of the chip. Its stacking technique is particularly significant, however, for the combination of different chips; for example, of memory chips and logic chips. During the processing of the silicon wafer it is possible, for instance, to carry out cost-effective production of one kind of chips which are subsequently stacked atop one another and electrically contacted.
In accordance with the invention, the stacking can be done, for example, either by stacking wafers on wafers or by stacking chips on wafers. In each case, when processing is at an end, the individual stacks require separation from one another, by sawing, for example.
Stacking takes place in accordance with the invention by means of bonding with the poly-o-hydroxy amides of the invention. Bonding with these high temperature stable, permanent adhesives of the invention has the advantage that these adhesives can be processed by the technique of spincoating, which is a standard operation in microelectronics. Spincoating and bonding may be followed without problems by other clean-room operations such as sputtering or etching. In accordance with the state of the art, stacking is frequently accomplished by means of soldering. In this case, however, the implementation of downstream processes in chip manufacture is no longer possible in a xe2x80x9cclean roomxe2x80x9d, since the soldering operation is cleanroom-incompatible.
The present invention also relates to a process for adhesively bonding materials and components. In principle the process comprises the steps of:
applying poly-o-hydroxy amides of the present invention to the area or areas of the materials or components to be bonded;
contacting the areas to be bonded with one another, and
crosslinking the poly-o-hydroxy amides;
and optionally then baking to convert the poly-o-hydroxy amides into the corresponding polybenzoxazoles.
Crosslinking takes place, in accordance with the invention, preferably by means of temperature treatment, laser treatment, ultrasound or microwave treatment, with temperature treatment being particularly preferred. Crosslinking presumably takes place substantially with the participation of the terminal groups (endcaps) of the poly-o-hydroxy amides of the invention.
The temperature treatment takes place preferably at a temperature of 80-280xc2x0 C., with further preference 120-220xc2x0 C., for 1-30 min, preferably 2-10 min.
Contacting of the areas takes place preferably under an applied pressure of 0.5-10 N/cm2, preferably 4-6 N/cm2.
The optional subsequent baking step takes place preferably at a temperature of 280-420xc2x0 C., more preferably 320-400xc2x0 C., for a period of 20-120 min, more preferably 40-80 min.
The poly-o-hydroxy amides of the present invention are applied preferably in the form of a powder to the areas which are to be bonded and the powder is converted by heating into a melt which can be spread over the surface.
In accordance with a further, preferred embodiment the poly-o-hydroxy amides of the present invention may be applied as a melt, or in solution in an organic solvent, to the surface or area that is to be bonded, by spincoating techniques, spraying or spreading or brushing. In the case of solvent-based systems, drying is advantageous, and in many cases is also necessary.
Examples of suitable solvents for the poly-o-hydroxy amides of the invention include N-methylpyrrolidone, xcex3-butyrolactone, ethyl lactate, ethoxyethyl propionate, tetrahydrofuran (THF), cyclopentanone, cyclohexanone, ethyl acetate, isopropanol, ethanol, acetone or diethylene glycol monomethyl ether or mixtures thereof.
The PHAs are preferably in a concentration of 10-45% by weight, more preferably 20-35% by weight, based on the overall composition; the solvent is preferably in a concentration of 55-90% by weight, more preferably 65-75% by weight. By means of the PHA content it is possible to control the viscosity of the composition.
The solution of poly-o-hydroxy amides and organic solvent may optionally contain, based on overall composition, 0.1-10% by weight, more preferably 0.1-5% by weight, with further preference 0.5-3% by weight, of a crosslinker. By using crosslinkers it is possible to exert a positive influence over the cure behavior, strength, and thermal and chemical stability of the poly-o-hydroxy amides. Crosslinkers which can be used in this context include short-chain compounds containing at least two groups capable of addition polymerization or polycondensation. As crosslinkers it is possible to employ crosslinkers which are known in the state of the art. With preference it is possible in accordance with the invention to use the following compounds as crosslinkers: 
where R2 is as defined above.
In order to improve the adhesion properties, the (surface) areas to be bonded may be cleaned and/or roughened beforehand by means of plasma, solvent, sandblasting, flaming or brushing or other mechanical abrasion techniques. Dust residues can be removed, for example, by blowing with compressed air or washing in water.
In order to remove residues of oils and fats or other impurities, the components to be bonded may be treated with solvents such as, for example, acetone, alcohols, halogenated hydrocarbons or aromatic hydrocarbons.
It is further preferable, before applying poly-o-hydroxy amide, to apply an adhesion promoter to at least one of the areas to be bonded. By using adhesion promoters it is possible to enhance the wetting of the surface to be bonded and thus the adhesion of the poly-o-hydroxy amides to the surfaces relevant in microelectronics, such as silicon, silicon oxide, silicon nitride, tantalum nitride, glass or quartz, for example.
Adhesion promoters which are particularly suitable in accordance with the invention are silanes containing at least one alkoxy group. Preference is given to N-(2-aminoethyl)-3-aminopropylmethyldimethoxysilane; 3-aminopropylmethyl-diethoxysilane; 3-(N-allylamino)propyltrimethoxysilane or (3-acryloyloxypropyl)trimethoxysilane (see formulae). 
It is preferred to apply the adhesion promoter to one of the areas to be bonded and the poly-o-hydroxy amide to the other of the areas to be bonded.
It is also possible in accordance with the invention, however, first to apply an adhesion promoter to both areas and then to apply the poly-o-hydroxy amide to one or both of the areas to be bonded.
The adhesion promoter is preferably applied to the area to be bonded in a substantially monomolecular layer. The adhesion promoters are normally dissolved with a concentration of approximately 0.5% by weight in a mixture of methanol, ethanol or isopropanol with approximately 95% by weight and 5% by weight of fully deionized water, and used further in this form.
The adhesion promoter is preferably applied by spincoating. This technique is known to the skilled worker. The area to be bonded may also be dipped into the solution of the adhesion promoter and subsequently blown dry. The areas treated with the adhesion promoter should be adhesively bonded within two hours.
The process of the present invention is especially suitable for stacking chips and/or wafers with one another and bonding them.
One chip or wafer stacking process which is preferred in accordance with the invention comprises adhesively bonding a base wafer carrying functional chips to a second, thinned wafer, likewise carrying functional chips. In this case the thinned wafer is located on another, unthinned wafer, which only has a carrier function. After the two wafers carrying functional chips have been bonded, the carrier wafer is separated from the stack. This process is depicted in FIG. 1. First of all, the base wafer 1 is bonded by means of a temporary adhesive 2 to a carrier wafer 3. Subsequently, the wafer 1 is shaped on the carrier wafer 3 into a thinned wafer 4. Thereafter, a permanent adhesive 5 in accordance with the present invention is applied to the thinned wafer and baked. This is followed by bonding to the base wafer 6. Finally, the carrier wafer 3 is detached. This separation is carried out by detaching the temporary adhesive layer, under the effect of temperature, for example, which is located between the thinned wafer 4 and the carrier wafer 3. This second adhesive layer differs from the permanent adhesive of the present invention in particular in that it remains meltable and hence soluble following a temperature treatment. FIG. 1a shows the condition after the bonding of the wafer 1 carrying functional chips to the carrier wafer 3. FIG. 1b shows the situation after the thinning of the wafer 1 to give a thinned wafer 4 on the carrier wafer 3. FIG. 1c shows the condition following application and baking of the permanent adhesive 5. FIG. 1d shows the condition after bonding of a wafer 6 carrying functional chips by permanent adhesive 5, and FIG. 1e the condition after removal of the carrier wafer 3.
The chips lying one above the other on the two wafers can then be connected electrically to one another. In this case, it is possible first of all to etch the permanent adhesive, to clean the contact hole with cleaning solutions, and to fill it with a metal, such as tungsten, which can be done by means of a CVD process.
In the case of the chip and/or wafer bonding process of the invention, for example, a first wafer is spincoated with a poly-o-hydroxy amide, dried at 80-120xc2x0 C. and then baked at 200-350xc2x0 C. The second wafer is either treated in the same way as the first wafer or, more advantageously, is provided only with an adhesion promoter. In many cases it is sufficient for this wafer to have been cleaned with a solvent beforehand and baked at at least 200xc2x0 C. The two wafers are then bonded to one another at an elevated temperature (100-350xc2x0 C.). After bonding, the wafers are again baked at 300-430xc2x0 C. The duration of the bake is 15-60 minutes. This baking converts the poly-o-hydroxy amide into the corresponding polybenzoxazole.
The bonding process of the invention requires no plasma activation or plasma equipment. The adhesive layer is active for a number of hours; in other words, there is no risk of deactivation even in the case of operations which take a very long time. Accordingly, the process is also very suitable for bonding chips to wafers.
The wafers or chips bonded to one another in accordance with the invention have a permanent bond. This bond is unaffected either by high temperatures of up to above 500xc2x0 C. or by chemicals such as solvents, acids, bases or aggressive gases. This is very significant, because the bonded parts are interrupted in numerous processes such as etching, sputtering, chemical cleaning, etc. A further advantage lies in the fact that the poly-o-hydroxy amides absorb much less water than the polyimides known in the state of the art, and so the risk of blistering is much less. The chip stacks and wafer stacks show no stress-related cracks, flakes or creases, which is frequently a problem especially in the case of thinned wafers.
Overall it is a particular advantage of the present invention that by means of the invention it is possible to implement permanent and reliable stacking of wafers and/or chips, with the bonds being extremely stable with respect to temperature and chemical exposures and exhibiting substantially no mechanical defects.
The present invention further embraces articles comprising at least two parts which have been adhesively bonded using poly-o-hydroxy amides of the present invention. The articles in question are preferably bonded chips and/or wafers. These may be bonded to give stacks, which may contain, for example, up to 8, preferably 2-4, chips and/or wafers. Bonded chip and/or wafer stacks of this kind are one preferred embodiment of the present invention.
The present invention further relates to adhesive compositions containing 10-45% by weight, preferably 20-35% by weight, of a poly-o-hydroxy amide, 55-90% by weight, preferably 65-75% by weight, of an organic solvent, and optionally 0.1-10% by weight, preferably 0.1-5% by weight, more preferably 0.3-5% by weight, of a crosslinker, in each case based on the overall composition. It is preferred for the composition to contain no other constituents, i.e., to consist of the stated constituents. It is further preferred for the composition to contain no phenol compound additions, particularly not the phenol compounds (C) disclosed as being necessary in EP 807 852 B1. Furthermore, the composition preferably does not contain the organosilicon compounds (D) disclosed alternatively as being necessary in EP 807 852 B1.
The present invention is illustrated below with reference to examples, which are not intended, however, to restrict the scope of the invention.